1. Field of the Invention
The present invention relates to optical information transmission that uses an optical fiber, and more particularly to an optical wavelength add-drop multiplexer.
2. Description of the Related Art
The wavelength division multiplexed (WDM) optical transmission method is a very effective method for increasing the capacity of optical fiber communication. In this method, a plurality of optical signals, each of which has a wavelength different from the other, are multiplexed in one optical fiber to transmit information. The optical wavelength add-drop multiplexer is one of transmission equipments placed for respective nodes in optical fiber networks which carry WDM signals. The optical wavelength add-drop multiplexer uses a technology whereby from among WDM signals that are transmitted through an optical fiber, only an optical signal having a necessary wavelength is branched (dropped) to receive the signal, and an optical signal transmitted from this node is inserted (added) into the WDM signals. In the optical wavelength add-drop multiplexer, most WDM signals transmitted through an optical fiber can be transmitted “through” as light without conversion into electric signals. The optical wavelength add-drop multiplexer, therefore, has an advantage of being able to reduce, to a large extent, the number of optical transceivers required for each node. Above all, when the reconfigurable optical wavelength add-drop multiplexer (ROADM) uses an optical switch, or the like, it can change, if necessary, a wavelength that is added/dropped at each node. The reconfigurable optical wavelength add-drop multiplexer attracts a great deal of attention because wavelength paths can be set by flexibly changing a configuration of transmission equipment according to the future growth of a network.
FIG. 2 is a diagram illustrating a configuration of a typical conventional reconfigurable optical wavelength add-drop multiplexer 120. A WDM signal, which has been transmitted from a WDM optical transmission device such as another reconfigurable optical wavelength add-drop multiplexer, is inputted from an input optical fiber 101, and is then amplified by an optical amplifier 102-1. The amplified signal is then inputted into the reconfigurable optical wavelength add-drop multiplexer 120. This is an example in which an optical signal having 16 wavelengths is wavelength division multiplexed. A WDM signal is separated by a optical wavelength demultiplexer 104 into different paths on a wavelength basis. The separated signals are output to output fibers 106-1 through 106-16 of a optical wavelength demultiplexer. After that, for example, an optical signal having a wavelength λ1 passes through a 2×2 optical switch 107-1, and is then inputted into a variable optical attenuator 110-1. On the output side of the variable optical attenuator 110-1, an optical signal is branched by the optical coupler 112-1, and part of the optical signal is thereby inputted into the forward direction optical detector 111-1. Usually, an insertion loss of the variable optical attenuator 110-1 is automatically adjusted so that the optical power detected by the forward direction optical detector 111-1 is kept constant, variations in optical power among optical signals each having a specific wavelength are reduced. After that, the optical signal having the wavelength λ1 is inputted into the optical wavelength multiplexer 105 through the input fiber 115-1, and is wavelength division multiplexed with optical signals having the other wavelengths λ2 through λ16 again. Then, the multiplex optical signal is amplified up to a specified output level by the optical amplifier 102-2 before the signal is transmitted to other optical transmission equipment from the output optical fiber 103. Usually, an AWG (arrayed waveguide grating) element, an element in which a tandem connection between dielectric multi-layer coating and an optical fiber grating is made, and the like, are used as the optical wavelength demultiplexer 104 and the optical wavelength multiplexer 105.
The 2×2 optical switch 107 is a switch used to switch between an add-drop state and a through state. For example, if the 2×2 optical switch 107-16 is in the through state, an optical signal having the wavelength λ16 output from the output fiber 106-16 of the optical wavelength demultiplexer passes through the 2×2 optical switch 107-16 just as it is, and is then inputted into the optical wavelength multiplexer 105. On the other hand, if this switch is switched to an add-drop state, an optical signal having the wavelength λ16 output from the output fiber 106-16 of the optical wavelength demultiplexer is output from a drop optical output fiber 109-16. Accordingly, it becomes possible to receive this optical signal by an optical receiver 124 placed in this network node. Incidentally, an optical signal having the wavelength λ16 inputted from the optical add signal input fiber 108-16 is sent to a variable optical attenuator 110-16. As a result, an optical signal, which is transmitted from an optical transmitter 123 placed at this network node, can be added to a wavelength division multiplexing signal that is output from the output optical fiber 103.
Typical configurations of the reconfigurable optical wavelength add-drop multiplexer include not only the above-mentioned configuration, but also the broadcast and select type reconfigurable optical wavelength add-drop multiplexer as described in “R-OADM Architecture: Now You Can Control The Light”, Architectural White Paper, Tropic Network Inc., May 3, <URL: http://www.tropicnetworks.com/library/pdf/ROADM_White_Paper—May—03.pdf>, and the like. FIG. 3 is a diagram illustrating a configuration example of the conventional broadcast and select type reconfigurable optical wavelength add-drop multiplexer 130. Inside this reconfigurable optical wavelength add-drop multiplexer 130, part of a WDM signal inputted from the input optical fiber 101 is first branched by the optical coupler 116-1, and then the branched signal is further branched by the optical coupler 133-1 into a plurality of drop optical output fibers 131. In this example, the number of drop output fibers is 4. A WDM signal, the power of which is attenuated, is output as it is from each of these drop output fibers. When it is intended to receive part of a WDM signal at this network node, a wavelength-selective optical receiver 125 having a function of selecting an optical signal wavelength to be received is connected to each fiber. By use of the wavelength-selective optical receiver 125, an optical signal having a desired wavelength is selected so that this specific optical signal is received.
On the other hand, another optical signal, which has been branched by the optical coupler 116-1, is inputted into a gain equalizer 134. The gain equalizer 134 is an element having a function of eliminating an optical signal having an unnecessary wavelength by sufficiently attenuating this optical signal so that only optical signals having necessary wavelengths are passed through as through signals. For example, the gain equalizer 134 can be configured as shown in FIG. 4. In FIG. 4, a wavelength division multiplexed signal inputted from the input optical fiber 101 is branched by an optical wavelength demultiplexer 104 into different paths corresponding to the maximum number of wavelengths (in this example, 16). For example, an optical signal having the wavelength λ1 is inputted into the variable optical attenuator 110-1. Then, part of the optical signal is branched by the optical coupler 114-1 so that the branched optical signal is inputted into the forward direction optical detector 111-1. The power of an optical through signal can be kept constant when an insertion loss of the variable optical attenuator 110-1 is automatically adjusted so that the optical power detected by the forward direction optical detector 111-1 is kept constant. In addition, making a loss of the optical attenuator sufficiently large makes it possible to eliminate only optical signals having unnecessary wavelengths. After that, the optical wavelength multiplexer 105 multiplexes only through signals having necessary wavelengths, before outputting the multiplexed signal to the output optical fiber 103.
In the configuration shown in FIG. 3, optical add signals, which have been inputted into four optical add signal input fibers 132-1 through 132-4, are combined by the optical coupler 133-2 into one signal. After that, this signal is introduced into the optical coupler 116-2, where the signal is further combined with optical through signals output from the gain equalizer 134, and is then output to the output optical fiber 103.
Such a broadcast and select type reconfigurable optical wavelength add-drop multiplexer is characterized by the drop optical output fibers 131-1 through 131-4 and the optical add signal input fiber 132-1 through 132-4, each of which is branched or coupled by use of an optical coupler that is not wavelength dependent. Therefore, the broadcast and select type reconfigurable optical wavelength add-drop multiplexer has advantages that the optical drop signal output fibers and the optical add signal input fibers are all independent of the difference in wavelength and the assignment of a wavelength, and that it is not necessary to consider a wavelength of an optical transceiver to be connected.
The conventional optical wavelength add-drop multiplexer as described above, however, is not sufficiently provided with a mechanism for detecting a misconnection caused by a user, which is a large problem. In particular, a problem which has not been conventionally pointed out is that a wavelength error of an optical add signal cannot be detected.
For example, it is necessary to properly connect optical transmitters, which output wavelengths λ1 through λ16, to the optical add signal input fibers 108-1 through 108-16 of the conventional reconfigurable optical wavelength add-drop multiplexer 120 shown in FIG. 2 respectively. In a building or an equipment room, in which these optical transmission devices are placed, a worker connects between different devices by use of optical fiber patch cables, or the like. Therefore, there is a possibility that the worker will connect the optical transmitter 123 handling an invalid wavelength to the optical add signal input fiber 108, a wavelength of which differs from that of the optical transmitter 123, as a result of connecting an optical fiber by mistake.
A dotted line 121 in the figure indicates a path of an optical add signal having a correct wavelength (λ16). To add an optical signal having the wavelength λ16, if the optical signal is properly connected to the optical add signal input fiber 108-16, the optical signal pass through the optical wavelength multiplexer 105 as indicated by the dotted line. Then, its output light is multiplexed into a WDM signal before the WDM signal is introduced into the output optical fiber 103. To the contrary, if the optical transmitter 123 handling the wavelength λ3 which is improper is connected to the optical add signal input fiber 108-16, an optical signal travels along a path 122 drawn with a dash-dotted line. In this case, although the optical signal arrives at an input fiber 115-16 of the optical wavelength multiplexer 105, the optical signal cannot pass through the wavelength multiplexer 105. Thus, the optical signal is blocked here. Because the forward direction optical detector 111-16 is provided in the middle of the optical signal path, it is possible to make a judgment at this point as to whether or not an optical signal exists. However, both in the case of a correct wavelength (path 121) and in the case of an incorrect wavelength (path 122), it is judged that an optical signal exists. Accordingly, an erroneously connected wavelength cannot be detected.
Accordingly, firstly, it is not possible to detect the occurrence of a misconnection in a work site. Even if a communication line stops due to a trouble, it is difficult to solve the trouble, and therefore problems of time and cost arise.
Secondly, the conventional reconfigurable optical wavelength add-drop multiplexer 120 has a problem that because an optical add signal is judged to have been internally added, power settings of the optical signal, or the like, are improperly made, causing the signal degradation. Usually, the reconfigurable optical wavelength add-drop multiplexer manages the number of wavelengths of a transmitted optical signal. Further, the output power of the optical amplifiers 102-1 and 102-2 is increased or decreased in response to the increase, or the decrease, in the number of wavelengths so that the optical power of the other wavelengths being used is not influenced by the change of state such as add and/or drop of a signal light having a certain wavelength. This is how to prevent the signal light power from varying. If as a result of the wavelength error as described above, even the optical signal which is not actually output from the reconfigurable optical wavelength add-drop multiplexer 120 results in a misjudgment in the device that an optical signal has been detected, the discrepancy is produced between the number of wavelengths actually transmitted and the number of wavelengths managed in the device. As a result, the optical output power is improperly set in the optical amplifiers 102-1 and 102-2. Thus, if the optical signal power varies from a proper set value, there is a possibility that the following large problems will arise: a signal-to-noise ratio of a signal light is not sufficient; a nonlinear optical effect is caused by the excess optical power, resulting in the degradation; the optical power inputted into an optical receiver exceeds an allowable range, resulting in incapability of receiving, or a breakdown of a receiver; and the like.
The broadcast and select type reconfigurable optical wavelength add-drop multiplexer shown in FIG. 3 also has the first and second problems described above. However, the broadcast and select type reconfigurable optical wavelength add-drop multiplexer further has a third problem. To be more specific, if an operator connects by mistake an optical transmitter, which handles the same wavelength as that of an optical through signal being transmitted or that of another optical add signal, to an optical add signal input fiber, an optical signal being used for information transmission cannot be transmitted, causing a serious failure. For example, as indicated by a path 136 drawn with a dotted line, an optical signal having the wavelength λ2 is inputted from the input optical fiber 101, and passes through the gain equalizer 134 as an optical through signal, and is then output to the output optical fiber 103. In this case, if the optical transmitter 123 for handling the wavelength λ2 is connected to an optical add signal input fiber so as to transmit an optical signal, an optical add signal travels along a path 137 drawn with a dotted line. As a result, both optical signals interfere with each other at an output point of the optical coupler 116-2. Because these optical signals are modulated with information signals that differ from each other, there is a possibility that one, or both, cannot be received. However, the broadcast and select type reconfigurable optical wavelength add-drop multiplexer does not have a mechanism for detecting a wavelength error of such an optical add signal, and an interlock mechanism for avoiding a failure caused by a wavelength error before it happens, which is the problem. In general, with the object of judging whether or not an optical add signal exists, the optical couplers 112-1 through 112-4 are placed at the input optical fibers 132-1 through 132-4 so that the optical add signal is detected by the optical detectors 135-1 through 135-4 respectively. However, by use of this configuration, it is not possible to make a judgment as to whether or not a wavelength of the optical add signal is correct.